1. Field of the Invention
This invention relates to the field of semiconductor wafer fabrication and, more specifically, to an apparatus and method for obtaining nondestructive molecular characterization of manufacturing defects within a semiconductor topography using inelastic scattering of incident monochromatic radiation.
2. Description of the Relevant Art
Scrupulously clean wafers are critical for obtaining high yields in the manufacture of integrated circuits with submicron device dimensions. The small feature sizes and the thinness of layers deposited on the surfaces of semiconductor wafers during the manufacture of such devices makes the process extremely vulnerable to damage caused by manufacturing defects. These manufacturing defects can degrade the performance or reliability of produced devices, and may even render a significant number of devices formed upon contaminated surfaces of semiconductor wafers inoperative.
General categories of semiconductor wafer defects include particles, films, and structural defects. Particles are any tiny pieces of foreign material that have readily discernible boundaries. Sources of particulate contamination include dust in the air, lint from clothing worn by clean room personnel, particles present in processing chemicals, and particles generated by processing equipment. Film contaminants form layers of foreign material on surfaces of semiconductor wafers. Examples of film contaminants include solvent residues, photoresist developer residue, oil films, and metallic films deposited during procedures involving immersion of wafers in a liquid. Structural defects include scratches, mounds, dimples, stacking faults and slip lines, and typically do not differ in chemical composition from the material immediately surrounding them.
In order to produce devices with acceptable performance and reliability characteristics, manufacturing defects must be detected. Optical microscopy can be used to detect particles down to about 0.5 microns in diameter, as well as the presence of scratches and solvent residues on wafer surfaces. Automated wafer inspection systems, such as those manufactured by KLA-Tencor, are now widely used in the semiconductor manufacturing industry for detection of defects, particularly particles and structural defects. These systems employ various illumination and image processing techniques, such as laser reflection from unpatterned wafers or subtraction of images from repeated structures on a patterned wafer (die-to-die comparison). Output generated by KLA-Tencor wafer inspection systems typically includes a defect map of a wafer, containing information on the size and location of defects present.
Once the presence of a manufacturing defect is detected, further analysis of the defect is needed. Information on the structure and chemical composition of a defect may enable removal of the defect during the process and preservation of the performance of the devices being manufactured. More importantly, however, information gained from analysis of a manufacturing defect aids in identification and elimination of the defect source, so that future occurrences of the defect are prevented. In order to determine the origin of particular defects, it is often desirable to examine product wafers (i.e., wafers expected to yield operational devices) before and after selected processing steps. Nondestructive analysis techniques are therefore needed.
Currently used analysis methods usually provide information on either the physical structure or elemental composition of defects. Non-destructive techniques of structural analysis commonly used in wafer fabrication include optical microscopy and scanning electron microscopy (SEM). A scanning electron microscope directs a beam of primary electrons at the surface of a wafer and detects emitted secondary electrons in order to form an image of the wafer surface. Another structural analysis technique which is being increasingly used in semiconductor characterization is scanning probe microscopy (SPM). SPM comprises a family of techniques in which a probe is held extremely close to a surface and scanned with high resolution and accuracy (tenths of nanometers). Some interaction between the probe and the surface is then measured. In the case of scanning tunneling microscopy, for example, tunneling current is measured. The most commonly-used SPM technique in characterization of semiconductor fabrication is atomic force microscopy (AFM), in which the force between the probe and surface is measured. Typical applications include measurement of roughness, pinholes, and other topographical features on a wafer.
Nondestructive techniques commonly used for elemental analysis include Auger emission spectroscopy (AES) and X-ray fluorescence spectroscopy (XRF). Like SEM techniques, AES techniques involve directing a beam of primary electrons at the surface of a wafer. Instead of forming an image using detected secondary electrons emitted by atoms on the upper surface of a wafer, AES techniques measure the energy levels of the emitted electrons to determine elemental compositions of surface structures. In XRF techniques, a beam of primary X-rays is directed at the surface of a semiconductor wafer, and the energy levels (or corresponding wavelengths) of resultant secondary X-rays emitted by atoms of elements on and just under the surface of the wafer are measured. Atoms of elements in target materials emit secondary X-rays with uniquely characteristic energy levels (or corresponding wavelengths). Thus the elemental compositions of materials on and just under the surface of the wafer may be determined from the measured energy levels (or wavelengths) of emitted secondary X-rays.
Although the above-described techniques are useful for defect analysis in semiconductor manufacturing, none of them are capable of providing molecular identification of defects. For example, a technique such as AES or XRF might identify the presence of the elements silicon and nitrogen in a defect, but would not be able to determine that these atoms were in, for example, a silicon oxynitride layer having a particular chemical composition. Knowledge of the actual chemical compounds present in a defect can be extremely valuable in identifying the mechanism through which the defect was formed. This is particularly important in the case of deposition processes, in which precursor materials, such as tetraethyl orthosilicate (TEOS), often consist of large molecules which may participate in complex chemical reactions on the wafer surface during a deposition.
Infrared (IR) absorption spectroscopy is a commonly-used nondestructive technique for obtaining molecular identification of materials. IR absorption spectroscopy involves detecting molecular vibrations, or vibrations characteristic of atoms which are bonded together. Incident radiation which has the same frequency as a molecular vibration in the material is absorbed. The result of the measurement is typically a plot of transmitted radiation intensity versus wavenumber (reciprocal of wavelength) of the radiation, showing many transmission dips corresponding to vibrational mode frequencies. Coupling between vibrations involving different parts of a molecule results in a complex spectrum which provides a distinctive signature for the particular chemical compound and phase being measured. A very popular instrument for performing infrared absorption measurements is the Fourier transform infrared (FTIR) spectrometer. In an FTIR spectrometer, transmittance (or absorption) is measured at all frequencies of the spectrum simultaneously, using interferometry and Fourier transform techniques. This results in an ability to average many measurements in a short time and realize significant improvements in signal-to-noise ratio.
A problem arises with FTIR and other IR absorption measurements for analysis of defects in semiconductor fabrication, however, because of the spot size of the incident beam. Wavelengths of the vibrations used for identification of most chemical compounds are in the mid-infrared region, from approximately 2 microns to 25 microns. Because the wavelength of the incident radiation must match that of the vibrations to obtain an absorption spectrum, the incident radiation used in IR absorption measurements is also in the wavelength range of 2 microns to 25 microns. The spot size of a beam of radiation is related to its wavelength such that the lower limit of the spot size is on the order of the wavelength. Therefore, the area illuminated by the incident radiation in an IR absorption measurement, and the area from which the resulting absorption signal is collected, can be on the order of 25 microns in diameter. Because many semiconductor manufacturing defects are of submicron size, this illumination area is much too large for isolation of a particular defect for analysis. To be useful for analysis of submicron-sized defects, an illumination area having a diameter of no more than approximately one micron is needed.
Even the ability to obtain molecular identification of a small region surrounding a defect is of limited utility without a technique for finding and focusing on a selected defect. Manual searching for defects using a microscope is extremely time-consuming and tedious, and submicron defects may not be visible by optical microscopy. A scanning probe microscope obtainable from Digital Instruments, model Dimension 7000, with the capability to import defect maps from KLA-Tencor wafer inspection systems and move its scanning probe to the site of a chosen defect has been advertised. There is, however, no known apparatus by which semiconductor manufacturing defects can be found and analyzed by a method providing nondestructive molecular identification.
During fabrication of an integrated circuit, various materials are grown from, deposited onto, or introduced into a semiconductor substrate. The resulting semiconductor topography, including the substrate and the overlying layers, changes throughout the process. A defect formed during one step of a fabrication process may be buried during subsequent steps, so that examination of manufacturing defects should not be confined to analysis of the surface of the semiconductor topography.
It would therefore be desirable to develop an apparatus and method for finding and obtaining nondestructive molecular identification of semiconductor manufacturing defects. To be useful for analysis of submicron-sized defects, the apparatus should allow analysis of an area as small as approximately one micron in diameter. The apparatus should also have the capability to controllably analyze material below the surface of a semiconductor topography. Moreover, the apparatus must have the capability to locate and focus on a chosen defect for analysis.